1. Field of the Invention
The invention is a shiftable planetary transmission having a sun gear, at least one planet gear mounted on a carrier, and a rotatable ring gear, which can be coupled with a non-rotatable or a rotatable transmission component.
2. Description of the Prior Art
Planetary transmissions are known in many forms and are used advantageously in vehicle drives to achieve multiple stage reductions between a high-speed motor and a significantly slower output element, such as a vehicle wheel, because they save space. Planetary transmissions are also used in construction machines because they are shiftable under load and permit relatively high travel speeds during operation over the road. As a rule, the desired higher travel speeds cannot be obtained with the hydrostatic transmission which is required for operation of the construction vehicle at a construction site.
To obtain the different shift modes, coupling or brake plates are usually provided to connect the rotatable components of the planetary transmission with other transmission components which may be stationary or rotatable. For example, a stationary transmission component may be the transmission housing and rotatable transmission components may be the carrier or the sun gear of the planetary transmission.
If a rotatable ring gear is coupled with a rotatable sun gear, a direct drive results. Alternatively, rotation of the ring gear may be prevented by coupling it with the transmission housing to provide a braking mode.
With a driven sun gear and a stationary carrier, it is possible to increase the torque on the rotatable ring gear. With a driven sun gear and a stationary ring gear, i.e., the ring gear is coupled to the transmission housing, it is possible to increase the torque on the rotatable carrier. These two modes are reduction modes, with the latter variation (output via the rotatable carrier) used more frequently. If both the carrier and the ring gear are freely rotatable, the planetary transmission is in a free-wheeling mode.
The activation of the coupling or the brake plates is generally accomplished hydraulically, which requires significant structural and assembly costs. Furthermore, the arrangement for moving the plates contributes to the size of the planetary transmission.